Strange knocking noise

W

WesChapman

Member
1) vehicle model. Scout terra
2) production date/or year model. July 1978
3) two wheel drive or four wheel drive. 4x4
4) type of transmission (and transfer case if applicable). T-19 close. Dana 20
5) if 4x4, which axles front and rear. Dana 44 x2 3.73
6) which engine variation. 345 presumed original
7) which ignition system variation. Prestolite
8) which fuel system variation. 2bb swapped for 4bb thermoquad
9) which brake system variation. Oem
10) non-oem modifications present which May impact a response. Relocated coil to fire wall. Installed remote starter relay.

I have an intermittent knocking noise on cold start that goes away as the engine warms up. It's occasional and not very loud. I only hear it, if I'm standing in front of the Scout near the passenger side of the radiator. I first thought it was the fuel pump, but I crawled underneath with a piece of pvc pipe and listened. I can't hear the noise through my listening pipe when placed on the fuel pump. I can hear the noise clearly, if I place the pipe on the oil pan where it bolts up behind the crank shaft pulley. The rhythm isn't consistent, which seems to my shade tree mechanic mind, would not be a rod. I posted about this on binder planet months ago and allen e posted back that it could be cam bearings going bad allowing the timing gear to move. He said it happened to one of his rigs. When I had the oil pan off to replace the gasket, I didn't see any evidence of cam bearing failure, but, that was last year. (or was it the year before?) now this noise is intermittent; sometimes I can cold start the Scout for several cycles, and never hear a bad noise from the engine.
I've been concerned about my oil pressure due to low readings from the gauge, but the temp & fuel gauge read low also, which made me think cvr. However, allen e's comment nagged at me, so I bought a mechanical oil gauge. On cold start, I have 30 psi at cold idle and above idle the pressure rapidly climbs to 50 psi and holds there. At hot idle, there is 20psi and above idle, it rapidly climbs to 50psi and stays there. Unless I'm mistaken, those reading are within spec for a 345. Would I have these pressures if the cam bearings were failing? The valve train is very quiet on this motor, I only hear a faint ticking once in a while.

Note:tested the cvr and only getting 1.5v from it according to my analog volt meter.
 
When the #1 cam bearing on these motors dumps, that prevents oiling of the distributor gear and the timing gear set as it receives a timed "squirt" with every cam revolution. In turn the cam usually breaks right behind the gear mounting position or through the oil hole in the cam journal. No noise whatsoever other than the engine scrambling it's eggs internally. That is extremely rare and if that happens then all the cam bearings are going/gone and that means rebuild anyway.

I'd surmise that you have developed either a wrist pin rattle or a "piston slap" condition which are fairly common on one of these motors when at some time in the past it has been severely overheated due to a engine cooling issue, not from lack of lubrication. The motor can run for many more years with that annoyance, I have a 196 myself with the same affliction. The four cylinder engines are much more prone to developing piston slap than the sv engines, I personally think that is due to a difference in the crank throw/rod/piston thrust scenario that is characteristic of the I-4 design.

Remove the plug wire on the offending cylinder while the engine is running,...noise should diminish/go away as that piston/rod is simply coasting and not under combustion pressure. When ya connect up the plug boot again, the noise comes back. This is not a rod bearing going away or the motor would already be scrambled and oil pressure numbers would not be as nice as you describe.

These motors use a "pressed" or non-floating wrist pin, that means the wrist pin is pressed into the rod small end when the piston is assembled (or a hear/expansion process is used). No circlips used to retain the pin as found on full-floating piston/rod assemblies. So the actual holes in the piston for the pin have slightly elongated/worn due to overheating, the pin has not broken loose in the rod eye.

Piston "slap" is the result of wear/distortion on the thrust side of the piston skirt on the affected cylinder/piston combo. While some pistons of course do wear (along with the cylinder bore), when a noise issue comes on like this suddenly, that means the piston has seen a thermal event and the skirt has "collapsed" to a small degree resulting in excessive cylinder-to-piston clearance on that one assembly.

With a $5 engine stethoscope probed on the valve cover, just below a sparkplug boss and right at the head gasket parting line, I bet you can pin-point the noise/offending cylinder immediately. Probed at the pan, you will hear all sorts of mechanical noises that are perfectly normal.
 
I am seconding the piston slap opinion. I have a 266 v8 with almost the exact same "noise" as you are describing from the same area of the block. It's been running that way, not getting worse, for over 10 years. I just baby it a little when it's cold, and then romp it like any other rig after it gets up to operating temp.
Your oil pressure numbers look excellent.
The cvr should be putting out around 5.5 volts mean. That is the average is 5.5 with occasional surges to maybe 7 or 8 and as low as 3, but it should average around 5 to 5.5 with a boucing needle on a sweep(analog) meter.
 
Wow times flies. I didn't realize how long ago I posted this.
Thanks for the replies. Ever since reading the sonjamotor thread and a few other cam bearing threads, I live in constant fear my motor will succumb to failed cam bearings. In reality, the few parts of the motor I have opened up (valve covers, oil pan, valley pan) the engine is remarkably clean and free of sludge and gunk. At some point in her past, the ole girl musta had a good home.
There is evidence the radiator leaked badly at one point, so an overheated engine is very likely.
 
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